In the automotive field it is well known to maintain the voltage supplied to the on-board electrical system by the vehicle alternator at a predetermined setpoint value, independently of the engine speed or the power consumption of the equipment, by means of a regulating device referred to as a “regulator”.
Nowadays, automobile equipment manufacturers have developed high-performance alternators through the application of electronic power systems controlled by circuits using digital techniques, based for example on the use of microprocessors or microcontrollers.
Unlike the bi-metal regulators formerly used, which operated around a fixed regulation voltage independently of the operating conditions of the vehicle, modern electronic regulation devices take advantage of the processing capability of the electronics to receive variable setpoint values transmitted by an engine control unit so as to optimise the torque taken off in relation to the production of electrical energy required.
It can be appreciated that the alternator does not need to draw torque on starting the engine or during an acceleration in order to improve the performance of small-engined vehicles, or that the torque taken off can be reduced because the battery is already sufficiently charged.
In order to achieve this end by simple means, U.S. Pat. No. 5,767,636 describes a serial link for transmitting various commands to the regulator, in particular a command to stop the generation of power, as well as for transmitting discrete values of the duty ratio of the switching elements of the alternator excitation current corresponding to a discrete series of setpoint values.
However, given that a serial link is prone to reliability problems in the disturbed environment of the engine compartment, U.S. Pat. No. 6,137,247 filed by the same applicant proposes to replace it by a transmission with pulse width modulation (PWM).
The setpoint values of the regulator, comprised between 12 V and 15 V, are transmitted by pulses of predetermined period of which the duty ratio is between 10% and 90% for example.
To allow more precise control of the alternator, other operating parameters, in particular the excitation level, are transmitted also using pulses with variable duty ratios, but having periods different from those fixing the setpoint values.
Signals having duty ratios below 10% or above 90% % are not used as they are considered to be too sensitive to noise.
It is well known that a PWM transmission is a simple and robust alternative to serial transmission. However, the use of pulses of several different periods to increase the capacity of the channel appears to run counter to the initial objective of simplicity.
There thus remains a need for a simple PWM transmission making it possible to transmit both the setpoints of the regulator and other operating parameters of the alternator.